The present invention relates to furnaces, particularly to those furnaces known as calciners. In a calciner, heat from a heat source is transmitted to the material to be calcined. Calcination typically involves a chemical or physical change, such as oxidation or dehydration of a material. In the pigment industry, yellow iron oxide, exemplified as Fe.sub.2 O.sub.3 H.sub.2 O, is calcined (heated) to remove the water, resulting in red iron oxide, Fe.sub.2 O.sub.3. Another process of obtaining red iron oxide involves heating FeSO.sub.4 7H.sub.2 O (copperas) to form FeSO.sub.4 H.sub.2 O, and then calcining (roasting) the monohydrate in a rotary furnace to form Fe.sub.2 O.sub.3. The present invention concerns a calcining apparatus capable of producing iron oxide "tans," in which another metallic oxide, such as zinc oxide or magnesium oxide, is combined with a majority of iron oxide, under the influence of heat, to form a ferrite, exemplified as M(FeO.sub.2).sub.2, where M=Zn or Mg.
Prior art calciners are typically rotary kilns, as shown and discussed in Perry's "Chemical Engineers' Handbook" (4th Ed., McGraw-Hill), particularly pages 20-15 through 20-31 and FIGS. 20-24, 20-36, and 20-37. A burner, burning gas or oil, supplies hot combustion gases. The material to be calcined, in this case iron oxide, is fed into the upper end of the calciner. The heat from the combustion gases is conducted in the iron oxide. Rotation of the calciner retort exposes the iron oxide to the heat, thus calcining the iron oxide.
Prior art calciners have the following disadvantages:
(a) Temperature control of the calcinable material is difficult, leading to variations in product quality. PA1 (b) Uniform retention time of product particles is difficult, leading to variations in product quality. PA1 (c) The inner retort surfaces frequently becomes unevenly coated with a layer of iron oxide, of variable thickness, leading to uneven heat conductivity and resultant variations in product quality. PA1 (d) At the higher temperatures required for calcining "tans," maintenance is difficult and costly. PA1 (e) The system is inefficient, based on the energy requirement per pound of calcined product. PA1 an elongated, insulated chamber having a feed opening and a discharge opening, PA1 conveyor means for transporting calcinable material from the feed opening to the discharge opening, PA1 heating means mounted within the chamber operative to direct heat energy throughout the interior of the chamber, PA1 a muffle, or baffle, mounted within the chamber and extending along the path traversed by the calcinable material, said muffle being so positioned within the chamber that the chamber is divided into two elongated compartments effective to shield and protect the heating means from said material, and PA1 vent means in the compartment housing the heating means providing an outlet to the exterior of the chamber.
Typically, a calciner to produce "reds" operates in the temperature range of 700.degree.-1700.degree. F. (370.degree.-925.degree. C.), with a retention time of 9 to 18 minutes. Under these conditions, the calciner retort life is of the order of 6 to 11 years. On the other hand, a calciner for "tans" operates in the 1750.degree.-1850.degree. F. (950.degree.-1010.degree. C.) range, with a retention time of 18 to 36 minutes. Under these conditions, the calciner retort life is about 1 to 3 years.